FIG. 7 shows a conventional servo control system. When servo control of each axis is performed using a three dimensional spatial position command, a conventional servo control system 60 (such as one used for a laser beam machine and a machining center) instructs an axis command producing portion 50 to produce a motion command for each control axis necessary to effect a spatial position command and velocity command instructed by a machining program.
The motion command for each axis is output to a corresponding axis control portion, and the axis control portion drives a motor on the basis of the motion command. At this time, feedback is gained regarding the position, velocity and acceleration of the motor at sampling increments so as to compute actual velocity and acceleration of the motor.
The difficulty in using such a method of determining velocity and acceleration through the use of feedback monitoring is that it introduces an element of delay into the control method, since, by the time the control parameter is computed, the machine has passed beyond the discrete point which the analysis is intended to model. Therefore, the motion of the machine in each axis is not controlled on the basis of the state of the motion at a specific instant in time, but on the basis of the state of the motion at some previous sampling time. As a result, when machining is executed at high speed, or when the tool is forced to negotiate a particularly severe curved section of a workpiece, substantial errors can result from this delay. Thus, the machining operation can be difficult to properly control and machining tolerance errors can result. In the servo control system 60 shown in FIG. 7, for instance, the spatial position command PC (which represents the path in three dimensional space along which the tool moves), and the velocity command VC, are transmitted to the individual axis command producing portion 50 (in this case, a velocity override command OC can also be given to the individual axis command producing portion 50). Upon receiving these commands PC, VC, the individual axis command producing portion 50 produces the position command Dn once every sampling increment “s” for each axis Sn (n=1, 2, . . . , 5) to be controlled.
An axis control portion 51 for each axis Sn produces the velocity command and the acceleration command (or power command) necessary for servo control from the position command Dn of the axis Sn, and executes servo control for the particular axis through a power control portion 56, which controls the electric power provided to a servo motor M for the axis. Sn. In this way, the position command Dn is translated into a new tool position by position loop 52, velocity control is performed by a velocity loop 53, and acceleration control is performed by an acceleration loop 55.
But, as previously noted, since the velocity command and the acceleration command are produced on the basis of the state of the control axis at a specific point in time in axis control portion 51 of each axis Sn, the control of velocity and acceleration has an inherent delay element in this conventional servo control 60. The influence of this delay element is even bigger when using spline interpolation (or circular arc interpolation). Therefore, the movement of the working point of the tool, which is the composite movement of each axis, is not smooth and will include irregularities.
Besides, when a nonlinear element is encountered, it is necessary to limit the system to prevent sudden changes in velocity and acceleration that are beyond the limits of the tool, or which will introduce unacceptable machining error into the workpiece. With the conventional method, in which the velocity command and the acceleration command are produced from the position command which itself is based on a sample taken at a particular sampling time, then adequate control may be impossible. This results in an increase in error between the actual position of the tool and the commanded position of the tool. As a result, feeding irregularity (which is integration of the acceleration and position shift which is the integration of the feeding irregularity) occurs.
The object of the present invention is to provide a numerically controlled method capable of reducing feeding irregularity or position shift and which allows execution of curved face machining with high accuracy, taking the above-mentioned considerations and concerns into consideration.